In just nine days, voters in Scotland will head to the polls to vote on a referendum that could bring an end to the union of Scotland and England that began when James VI of Scotland also became James I of England due to the Union of the Crowns. When I last wrote about the referendum last month, it seemed as though the independence initiative would go down to defeat. The “Yes” vote had never really lead in the polls in the past, and the momentum seemed to be clearly shifting to the “No” side of the ballot. In the last several days, though, as the battle comes down to the wire, the outcome of perhaps the most important vote in the history of the United Kingdom is becoming less and less clear:

LONDON — The once-unthinkable prospect that Britain could be ripped apart this month with a vote for Scottish independence became bracingly real Monday after the campaign to keep the three-century-old union together was accused of panicking amid polls showing the referendum in a dead heat.

Just 10 days before the vote, the new surveys depicted a dramatically tightening race after months in which the “no” side appeared to hold a comfortable lead. Although both sides have questioned the accuracy of the Internet-based polls, the pro-independence camp immediately claimed the momentum.

Unionists, meanwhile, scrambled to agree on a plan for shifting power away from London and giving it to the Scottish government if the Scots choose to stay, with former prime minister Gordon Brown saying his Labor party would move aggressively to do just that.

But it was unclear whether the other major parties agreed with Labor’s plan, and the unionists were forced to spend Monday fending off accusations that they were desperate to stop a slide toward “yes.”

“They had become so overconfident. They never expected to be in this position,” said Iain Docherty, a public policy professor at the University of Glasgow. “So they’re improvising, and they may make things worse for themselves.”

Docherty said the unionists’ moves in the coming days are likely to be a “make-or-break moment” for the future of Scotland — and for the United Kingdom.

The potential consequences of the Sept. 18 vote are vast for a nation that could be hard-pressed to continue to call itself Britain if a third of its land mass disappears beyond a foreign border. Britain’s military, economy and politics could be reshaped with the vote — as could its image as a world power, even if that status has been long in decline.

“In terms of its impact on the British self-image, it would be analogous to the loss of empire in the 1940s, ’50s and ’60s,” said Tony Travers, a government professor at the London School of Economics.

The independence of Scotland, he said, could carry even more of a punch because Scotland has been an inextricable part of Britain’s identity for centuries. “Really, there are no parallels for it,” he said. “There’s no doubt that if the Scots vote to leave, it would be a spectacularly traumatic experience for the British political class.”

Analysts say that Scottish secession probably would bring calls for the resignation of British Prime Minister David Cameron, with less than a year to go before he is due to face voters in a general election.

The Tory leader’s main rival in that contest, Labor chief Ed Miliband, also could suffer if the vote swings to “yes”: Scotland reliably votes for Labor in the general election, and without Scottish seats in play, the Conservatives could win British elections they otherwise would lose.

Although the prospect of Scotland’s secession has been real for two years, ever since Cameron and Scottish First Minister Alex Salmond agreed on terms for a referendum, the chances seemed remote until recent days. Among the political class in London, confidence was high that the Scots would choose to stay.

But the latest surveys have abruptly shifted the mood. One, by the polling firm YouGov, showed the independence camp with a lead for the first time — albeit by an extremely narrow margin of 51 percent to 49 percent when voters who answered “don’t know” were excluded. Another survey, by Panelbase, essentially flipped those results, showing 52 percent favoring “no” and 48 percent siding with “yes.”

Both polls were based on Internet surveys, the accuracy of which has been questioned by the campaigns and by polling experts.

John Curtice, a politics professor at the University of Strathclyde, said that although the results appeared to show a shift toward “yes,” more data is needed. With turnout expected to be exceptionally high — perhaps 80 or 90 percent of Scotland’s more than 4 million eligible voters — the outcome will be unusually difficult to predict.

“The truth is that the polls have surrounded the referendum in a measure of uncertainty throughout the campaign,” Curtice said. “It may be that we’ve always been in a close race, and it’s going to continue to be close.”

In contrast to those polls, another poll that was conducting using more traditional face-to-face interviews finds a much murkier situation, with 39% opposed to independence, 38% in favor, and 23% undecided. Notwithstanding that poll, as well as all of the reasons why Scottish independence would be a bad idea for Scotland economically, those in favor of the Union are not taking any chances. Today, for example, Prime Minister’s Question Time in Parliament has been postponed as Prime Minister Cameron, Deputy Prime Minister Clegg, and Labour Party leader Ed Millband all head north for campaign events. Additionally, the United Kingdom is basically promising Scotland more autonomy if it votes against independence:

LONDON — Shaken by polls showing momentum shifting toward independence for Scotland, the British government will offer proposals for greater political and fiscal autonomy for the Scots if they vote to remain within the United Kingdom in a referendum on Sept. 18, George Osborne, the chancellor of the Exchequer, said on Sunday.

The narrowing polls have caused considerable anxiety among politicians and business leaders, driving down the value of the pound and raising questions among investors about the stability of the economy and the fate of the current British government.

The vote, which could bring an end to the 307-year union between Scotland and England, is also regarded as important to the future of the British prime minister, David Cameron. As leader of what is still formally known as the Conservative and Unionist Party, Mr. Cameron, already facing internal divisions over Britain’s membership in the European Union, may not survive politically if Scotland votes to break away from the United Kingdom in a referendum that he negotiated with the Scottish National Party and its leader, Alex Salmond.

On Sunday, Mr. Osborne, a close ally of Mr. Cameron’s, responded to the tightening race by promising more powers to Scotland if it votes no.

“More tax-raising powers, much greater fiscal autonomy,” Mr. Osborne told the BBC. “More control over public expenditure, more control over welfare rates and a host of other changes.”

The plan will be revealed “in the next few days” after the government gets agreement from all three major parties in the British Parliament, including the Labour Party and the Liberal Democrats, Mr. Osborne said.

The plan will be revealed “in the next few days” after the government gets agreement from all three major parties in the British Parliament, including the Labour Party and the Liberal Democrats, Mr. Osborne said.

“Then Scotland will have the best of both worlds,” he said. “They will both avoid the risks of separation but have more control over their own destiny, which is where I think many Scots want to be.”

That position is sometimes known as “devo max,” or maximum devolution, an alternative that Mr. Cameron did not allow Mr. Salmond to put on the ballot. Instead, Mr. Cameron insisted on a simple yes or no vote for independence. In return, he allowed Mr. Salmond, Scotland’s first minister, to extend the vote to people ages 16 and over but limit it to those who are registered in Scotland, which excludes many Scots living and working elsewhere in Britain.

Mr. Salmond dismissed Mr. Osborne’s proposals on Sunday as “a panic measure.” Mr. Salmond’s deputy, Nicola Sturgeon, calling the new polls a “very significant moment” in the campaign, said the offer of new powers had come very late.

“I don’t think people are going to take this seriously,” she told Sky News. “If the other parties had been serious about more powers, then something concrete would have been put forward before now.”

It’s not surprising that Salmond would react negatively to the proposal with this prospect of the independence he has been advocating for year now seemingly so close, but the real question will be how the voters in Scotland will react to something such as this.  Even with the recent uptick in support for independence, it seems apparent that there’s a great deal of reluctance on the part of Scottish voters for the idea of independence, perhaps in no small part because while the idea of an independence Scotland may appeal to some sense of national pride, the realities of that independence are less than appealing. By every reasonable measure, it seems apparent that independence would be an economic disaster for the Scots, especially in the short term. Scotland would quickly become one of the poorest nations in Europe and, due to that fact alone, may find gaining membership in the EU and other continental organizations to be more difficult than independence advocates seem to think it will be. An independent Scotland would also no longer receive the subsidies that the rest of the United Kingdom’s send into the region, may not benefit from the North Sea oil fields as much as anticipated, and would find itself outside of NATO and not in the best position to make the case that it could contribute to its own defense as much as members are expected to.

On the other hand, as Neil Irwin points out, it seems clear that Scottish independence is being driven by things other than economics:

What’s all the more remarkable about this possible secession is that major, specific grievances over public policy between Scotland and the rest of Britain are hard to identify. This isn’t like the Southern chunk of the United States seceding in 1860 because it was committed to slavery and the North was against it.

Sure, the pro-independence leaders make some promises about improving social welfare benefits like improved public child care for young children. But that doesn’t square particularly well with the fact that Scotland has been a net drain on the rest of British taxpayers for the last generation; it has received greater benefits than it has paid in taxes. And whatever long-term arrangements are reached over thorny issues like oil rights, divvying up public debt and currency arrangements between the Independent Scotland and the “rump U.K.,” as British commentators have been calling the possible post-secession nation, there is sure to be a heavy transition cost and damage to commerce in the near term.

Many Scots feel like they have more to gain from governing alongside people who look like them and talk like them than they have to lose from no longer being part of a bigger, more powerful nation. A video posted by the pro-independence campaign captures a bit of this. Amid soft-focus images of beautiful Scottish landscapes and charming-looking Scots going about their day, a woman holding flowers says, “Independence. It’s what we all want in our lives. So why shouldn’t our country be independent too?”

Whatever the impact of the latest moves out of London, it’s clear that we’re likely to see a rather united push from both the government and the opposition in favor of a “No” vote in the coming week. If independence does win, then it seems likely that David Cameron’s government would not survive. The stakes are likely higher for Labour, though, given the fact that a significant part of their presence in Parliament comes out Scotland. If Scotland leaves the United Kingdom, then Labour loses all of those seats and may find itself sitting on the sidelines while the Tories and the new, and politically ascendant UK Independence Party fight it out for control of Parliament for many years to come.